The present invention relates to a method for gaseous carburization of steel, where, in a carbon-rich atmosphere, an article that is to be carburized, in a first carburization phase, is exposed to a carbon supply or charge that is as great as possible, at the soot or black limit, and, in a subsequent diffusion phase, a lower carbon charge that corresponds to the desired carbon content at the surface of t he article is established, with carburization being regulated via the two target values carbon content at the surface and depth of carburization.
The basic objective during the carburization of steel is to undertake an enrichment of carbon in a surface layer of the workpiece or article that is to be carburized in such a way that in the atmosphere about the article, especially in a furnace atmosphere, an increased carbon supply or charge exists at an appropriate temperature. The carbon from the atmosphere diffuses into the article, and within the article itself diffuses from the surface layer into the interior of the article, where, at a distance from the surface of the article up to a carburization depth of approximately 3 mm the carburization is perceptible by a carbon content that is clearly greater than that of the base article. The carbon distribution in the article from the surface to the core can be diagrammatically illustrated in the form of a so-called carbon profile. The objective is, by carrying out the process, to achieve an S-shaped carbon content curve having a predetermined surface carbon content and as wide a horizontal zone at the surface as possible.
It is known, for the carburization of steel, to coordinate the performance or execution of the process with the two target values carbon content at the surface C.sub.R and depth of carburization A.sub.t. Practically speaking, this is carried out in that initially, in a first treatment phase, the carbon level is set at a value just below the soot or black limit and is held there (carburization phase), and subsequently, in a final stage (diffusion phase), a lower carbon level is used to achieve the desired carbon profile. For this purpose, computers are used which, at least at certain time intervals, are supplied with the process parameters that are noteworthy for the carburization process, such as temperature, oxygen potential, carbon level, carbon flow, and the like, with these parameters or values being utilized as control parameters for regulating the carburization process. A drawback with such a manner of performance or execution of carburization that is coordinated merely with the two target values carbon content at the surface C.sub.R and depth of carburization A.sub.t is that the desired S-curve shape of the carbon profile in the surface layer of the article that is to be carburized cannot be exactly and reproducibly established. The reason for this is that the point in time at which a changeover or shift is made from the carburization phase to the diffusion phase has a decisive impact upon the shape of the carbon profile in the article. If this point in time is selected too early, a carbon profile is obtained that drops off very rapidly from the surface toward the inside, whereas as if the point in time is selected too late, carbon profiles having an over-carburization (shown as a bulge or hump in a graph) occurs. It has been attempted, via multiple simulated calculations, to determine the optimum shifting time that leads to the desired S-shaped carbon profile. Despite this additional calculating and testing effort, deviations, especially where disruptions in the carbon charge occur, cannot be avoided in practice, since the regulation of the carburization process takes into account only t he target values carbon content at the surface C.sub.R and depth of carburization A.sub.t.
As a further development of this process for gaseous carburization of steel parts, German Patent No. 31 39 622 Winning dated Apr. 21, 1983 discloses slowly and slightly lowering the carbon level in the carburization phase via an intermediate phase that precedes the final phase, and continuing to do so until that point in time at which the carbon content at the surface C.sub.R has achieved a certain limit C.sub.R max. This limit is determined by the onset of carbide formation.
Despite this control value C.sub.R max that is added to the performance or execution of the carburization phase to avoid a carbide formation, this measure does not resolve the aforementioned problem of determining the correct point in time in which to shift from the carburization phase to the diffusion phase. Furthermore, under-carburization and over-carburization can occur, and there is no guarantee that the desired S-shaped carbon profile will be established.
It is therefore an object of the present invention, independent of the carbide limit, to provide a regulation of the carburization process via which it is possible to achieve in a straightforward manner that is reliable and reproducible, the desired carbon content curve (carbon profile) in an article. In particular, the present invention, via a two-stage manner of performance or execution of the process, make it possible to set the ratio of carburization time and diffusion time in such a way that as an end result the desired carbon content and case-hardening depth are achieved, and in so doing the carbon content curve receives a surface zone that is as horizontal as possible.